I've been using my Thwack! v3 for like 6 months, and I have NOT been gentle with it. It has held up amazingly well. I was using it to whack 608 bearings into holes designed to receive them (barely), and the head blew off. the handle failed right where it meets the head. TBH it's exactly the point you'd expect it to fail, and it sheared clean and smooth along the layer line.

I had used linear layers for my 100% infill, and that was a problem. So I increased the size of the fillet at the head/handle interface by a huge amount (I guess you can still call it a fillet), and I changed the layers to Archimedean chords. When I went to print it I suffered the same problem I had the first time. I didn't want to use rafts to improve the look, but It always broke free of the plate in the upper part of the print. So I removed the (I have to admit) cool direction arrows to increase the plate contact area, and I got further up but still failed, so I had to enable rafts. I used two layer rafts.

The result is a fair bit beefier Thwack! Which should stand up to the punishment you know I'm going to put it through,... and deal out to things.

So for printing, I did 8 walls, 100% infill using Archimedean chord layers, I printed it head down with 2 layer rafts . I also doubled the normal Z hop. You know, just to be sure. And I finally managed to print it reliably on my Creality Ender 3 V3 SE.